UWL: sort does not work for new attributes from the workflow container

Hi,

with the help of the UWL cofiguration wizard, I have defined some custom attributes for an individual task.

In UWL I can select my new view and get the new attirubutes. But I can sort any of these new columns?

For example when compare this with the "standard"subject field, the cursor is changing and I can sort that field:

This is XML coding I got generated:

  <CustomAttributes>

        <CustomAttributeSource id="WEBFLOW_CONTAINER" objectIdHolder="externalId" objectType="WebFlowContainer" cacheValidity="default">

          <Attribute name="Vendor_name" type="string" displayName="Supplier"/>

          <Attribute name="LV_Total_value" type="string" displayName="Total"/>

          <Attribute name="LV_CC" type="string" displayName="Comp. Cd."/>

        </CustomAttributeSource>

      </CustomAttributes>

      <Actions>

Is there something I have to add manually to get the sort functionality?

Thanks for any comments.

BR Jochen